The present invention relates to acoustical panels; and more particularly, it relates to an improved acoustical wall panel. Acoustical panels of the type with which the present invention is concerned have particular utility in "open plan" offices and schools. Open plan systems do not use conventional floor-to-ceiling walls to separate rooms--rather, individual wall panels are ganged together to define these areas. The height of the wall panels may vary, for example, in the range of 5-7 feet, and the widths may be 18-48 inches. Such panels need not be secured to the floor, and they terminate short of the ceiling.
Open plan office and school systems have received increased acceptance during recent years because of the ease of construction, relatively low cost, and flexibility. In an effort to further acceptance of these systems, attempts have been made to incorporate acoustical designs into open plan panels.
A typical acoustical panel for an open plan system which is used throughout the industry today is a glass fiberboard comprised of a plurality of layers of glass fiber and having a density of approximately six pounds per cubic foot. This panel does not absorb as much sound as is desired in the range of 500 Hz to 4,000 Hz.
Human perception of speech in the range of 400-8,000 Hz (sometimes referred to herein as the intelligibility range) has a disturbing effect if it is present in ambient noise because it is in this frequency range that intelligence is carried--such as the sounds of vowels and consonants. Thus, if sound is not absorbed in this range, it is distracting to a person who perceives it. Office machines are a source of noise in a band around 500 Hz, and the human voice is a source of noise in the range around 4,000 Hz. Thus, a panel which is deficient in absorbing incident sound at these frequencies is not an effective material for use in open plan systems.
Another disadvantage of the one-inch, six-pound density glass fiberboard is that the surface density is high enough that high frequency sound does not penetrate the surface efficiently--rather, it has a tendency to reflect, especially when the angle of incidence is acute (0 degrees to 45 degrees). This is sometimes referred to as to "flanking" angle, and it is particularly important in open office systems where a single wall extending in one plane may define two sides of adjacent rooms, with a separating wall extending perpendicular from it. If the separating wall is spaced from the long wall (for example, to form a door opening), then higher frequency sounds have a tendency to skip off the longer wall and penetrate into an adjacent room. Since these higher frequency sounds are in the intelligibility range, they become acoustical noise to the observer.
Another important aspect of an acoustical panel for offices and the like is that they not be too efficient in absorbing sound at frequencies below 400 Hz. The reason for this is that it has been found desirable to permit a certain amount of low frequency ambient sound for psychological reasons. These sounds are present as background and do not disturb or command the attention of one who perceives them. Rather, they have a quieting or reassuring effect provided they are not of such intensity as to command attention.
The present invention provides an acoustical wall panel having a core comprised of a plurality of layers of glass fiber which are pressed together under the application of heat and pressure. As many as three layers of the material, originally having a density of one pound per cubic foot, are laminated together to form a board having a nominal thickness of one inch. A resulting density variation from the front surface of the board to the rear surface ranges from approximately two pounds per cubic foot at the front suface to approximately six pounds per cubic foot at the rear surface. Further, a plurality of cavities are formed on the front surface. These may be provided by cylindrical projections (rods, stubs or pins) on one surface of the mold.
The board is provided with an impermeable back membrane or septum which, in the illustrated embodiment, is a sheet of aluminum having a thickness of 0.001 in.
The front of the panel is covered with fabric or other material which may be decorative, but also permits the sound pressure wave to enter the cavities formed in the front surface of the board.
The side walls of the cavity have a generally circular cross section which increases from the bottom of the cavity to the outer surface of the board to provide a smooth conformation from the bottom of the cavity to the outer surface of the wall. The cavities function as resonators to confine high frequency sound energy that enters through the permeable fabric covering until it is absorbed or attenuated by the air in the cavities. The pattern of placement of the cavities (preferably in a square or slightly diamond-shaped pattern) is such as to "tune" the board to enhance coupling of incident sound pressure waves into the cavities over a selected range of frequencies in the intelligibility range. For example, in a preferred embodiment, there are two spacings between cavities. One may be thought of as taken along the diagonal of the grid pattern, and the other is taken along a side. These two dimensions correspond to two quarter-wavelengths of sound in the lower portion of the intelligibility range so as to increase sound absorption. This increased absorption continues to the higher end of the intelligibility frequency range where the absorption is further enhanced by preserving the rough surface texture of the panel and by increasing the effective surface area due to the formation of the cavities on the outer surface of the panel.
By forming the glass fiber according to the present invention, the texture of the lower density layers is preserved at the outer surface of the inner or core material (namely the glass fiber). Thus, the core has a density of approximately two pounds per cubic foot at the surface, but it increases in the direction from the front surface to the rear surface, until it attains a density of six pounds per cubic foot at the rear surface. This density variation need not be a uniform, gradual increase in density, rather, it has been found that it increases from two pounds to approximately four pounds and then to approximately six pounds per cubic foot. The lower density material is efficient in surface-absorption of high frequency energy. The four pound per cubic foot density is effective to absorb the intermediate range (in the neighborhood of 400-500 Hz); and it is located at a position where the intermediate frequencies have greater penetration. Finally, the innermost section, having a density of approximately six pounds per cubic foot, is effective in absorbing the lower frequencies in a controlled manner. The septum acts as a barrier to prevent transmission of sound pressure waves.
Thus, by forming the core of the composite board in the manner described and by using the materials and density indicated, the texture of the outer surface is preserved for enhancing absorption of higher frequencies.
Further, the dimpled structure of the outer surface has a two-fold effect on incident sound at a flanking angle (that is, an included angle of incidence of 45 degrees or less). Considering that a sound pressure wave is transmitted with a generally spherical wave front and that the portion of the curved side wall of a cavity that the source of sound sees along the panel changes continuously, and further considering that the placement of the cavities is designed for particular frequency ranges, the first effect is that the source of sound or noise "sees" different portions of the curved cavity walls, and therefore at least some of the sound wave is incident to the cavity wall at a perpendicular angle, at which absorption is greatest. Secondly, any reflected sound is reflected at continuously varying angles because the angle of incidence changes for each cavity. This has the effect of dispersing the incident sound, causing it to lose its articulation and become less distracting.
The present invention thus provides an acoustical panel which has a frequency absorption characteristic which is better suited for use in an open plan setting in that it exhibits a frequency absorption characteristic which has high absorption for the higher frequencies which are perceived as noise by a human, which reduces the transmission of high frequency noise through flanking, yet which permits a controlled amount of low frequency or background noise for psychological assurance.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.